metode gravity
TRANSCRIPT
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Survei gravity digunakan untuk memetakan batuan dengan densitas yang berbeda
Survei skala regioonal :Memetakan intrusi yang berpotensi
mineralisasi emas kadar rendahPemetaan struktur basementMelokalisasi intrusi ultramafik
Survei detil:Eksplorasi sulfida & kimberlite
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Exploration With Gravity Gravitational Acceleration Measuring Gravity
Relative Measurements Pendulum Gravimeter
Absolute Measurements International Gravity Standardization Net 1971
Adjusting Observed GravityVariation As A Function Of LatitudeCorrecting For The Latitude EffectElevation Corrections
The Free-Air Correction The Bouguer Correction
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Elevation Corrections The Bouguer Correction The Terrain Correction The Isostatic Anomaly
Field Procedures Base Stations Elevations and Horizontal Positions Reduction Densities Survey Procedure
Gravity Effects of Simple Geometric Shapes Sphere Horizontal Cylinder Vertical Cylinder Inclined/Vertical Rod Horizontal/Faulted Sheet
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Analyzing AnomaliesRegionals and Residuals
Trend Surfaces Upward/Downward Continuation Second Derivatives
Analyzing AnomaliesRegionals and Residuals
Filtering
Gravity InterpretationHalf-Maximum TechniqueSecond Derivative Technique
Applications of the Gravity Method
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Shape: Oblate spheroid (that mean the earth bulges at the equator, and is shorter at the poles)
Why? Balance of forces: centrifugal due spin and gravity inward -- shape change so that a plumb bob will point perpendicular to the Earth’s surface
Less oblate during ice age due to unloading of equatorial regions from the accumulation of ice at poles
Equatorial radius: 6378.16 km Polar radius: 6356.77 km
Flattening of the earth: (Re-Rp)/Re=1/298.247
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Metode gravitasi adalah metode survey geofisika yang mengukur densitas batuan bawah permukaan bumi.
Metode ini didasarkan pada hukum Newton mengenai gravitasi.
Percepatan gravitasi bumi adalah gaya yang dialami oleh satu satuan massa akibat tarikan bumi.
Secara garis besar distribusi rapat massa atau densitas di dalam bumi menyebabkan percepatan gravitasi yang diukur di permukaan bumi bervariasi terhadap posisi.
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Metode gravitasi merupakan metode geofisika yang didasarkan pada pengukuran variasi medan gravitasi.
Pengukuran ini dapat dilakukan dipermukaan bumi, di kapal maupun diudara.
Dalam metode ini yang dipelajari adalah variasi medan gravitasi akibat variasi rapat massa batuan di bawah permukaan sehingga dalam pelaksanaannya yang diselidiki adalah perbedaan medan gravitasi dari suatu titik observasi terhadap titik observasi lainnya.
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Metode gravitasi umumnya digunakan dalam eksplorasi jebakan minyak (oil trap). Disamping itu metode ini juga banyak dipakai dalam eksplorasi mineral dan lainnya.
Prinsip pada metode ini mempunyai kemampuan dalam membedakan rapat massa suatu material terhadap lingkungan sekitarnya. Dengan demikian struktur bawah permukaan dapat diketahui. Pengetahuan tentang struktur bawah permukaan ini penting untuk perencanaan langkah-langkah eksplorasi baik minyak maupun meneral lainnya.
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Hukum Gravitasi Newton
rr
mmrF
20
g( r )=G(m2/r2 )
= F( r)/m1
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Gaya berat total bumi adalah sekitar 983.000.000 µGals. Radius equator bumi > 21 km pada radius kutub memberikan perbedaan sekitar 5.000.000 µGals. Perbedaan tinggi antara gunung tertinggi dan laut terdalam memberikan perbedaan sekitar 3.000.000 µGals. Struktur geologi regional memberikan perbedaan sekitar 100.000 µGals. Efek pasang surut bulan dan matahari sekitar 300 µGals. Kontur peta gaya berat mikro adalah 10 µGals. Sedangkan, akurasi gravimeter 1µGals.
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Satuan percepatan dalam SI = ms-
2
1Gal = 1 cms-2 = 10-2 ms-2
1mGal = 10-3 Gal = 10-5 ms-2
1µGal = 10-3 mGal = 10-6 Gal = 10-8 ms-2
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Koreksi ApunganTujuan: menghilangkan perubahan kondisi alat
Driftstation = x ( Tstation – TA1 )
A 1 2A 3A 4A
12
12
AA
AA
TT
GG
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Koreksi Pasang SurutTujuan: menghilangkan pengaruh gaya tarik matahari dan bulan terhadap bumi. Untuk melakukan koreksi ini digunakan Longman formula.
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Koreksi LintangTujuan: Mengkoreksi gaya berat di setiap lintang geografis karena gaya berat di setiap lintang tersebut berbeda. Perbedaan gaya berat tersebut disebabkan karena bumi yang berbentuk ellips.
2sin0000059,0sin0053024,01846,978031 22ng
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Koreksi KetinggianKoreksi ini ada dua yaitu: Koreksi BouguerKoreksi Bouguer: koreksi akibat
perbedaan ketinggian tanpa menghilangkan pengaruh massa di bawahnya.
Koreksi Udara BebasKoreksi Udara Bebas: koreksi akibat perbedaan ketinggian sebesar h dengan mengabaikan adanya massa yang terletak diantara titik amat
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Terjadi karena perbedaan elevasi antara titik pengamatan
Perbedaan elevasi terjadi karena perubahan topografi permukaan bumi pembacaan gravitymeter berbeda
Perlu dilakukan koreksi elevasi Koreksi elevasi mengasumsikan massa
yang terdapat pada titik observasi (elevasi tinggi – titik B) berupa slab dengan densitas dan ketebalan sama
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Spinning Earth -> centrifugal force, equatorial bulgecentrifugal force => less g at equator, no effect at polesequatorial bulge (~elliptical)
more mass near equator => g increasesr larger at equator => g decreases
Dependence of g on latitude ()
g() = 978032(1 + 0.0052789 sin2 – 0.00000235sin4) mgal
Elevation change r -> r + h => g decreases (“free air” effect)Free air correction:
g(r+h) = g(r) + (dg/dr) h
dg/dr = -2g/r = -0.307 mgal/m
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Koreksi MedanTujuan: menghilangkan pengaruh efek massa
di sekitar titik observasi.
22222112 LrLrrrGg
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h in feet!
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In general:
g = gobserved – gtheory
Free Air theory:
gFree Air = g(,h) = g() – 0.307 h
Free air anomaly:
gfaa = gobserved - gFree Air
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Anomali BouguerNilai anomali Bouguer lengkap dapat diperoleh dari nilai anomali Bouguer sederhana yang telah terkoreksi medan.
TBSBL ggg
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Approximate as a sheet mass:gBouguer=gFree Air + 2Gh;
for = 2.67, 2G = 0.112 mgal/m
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Gravitymeter
1. Fully digital reading system2. Automatic Leveling system 3. High accuracy (0.5Gal)4. Automatic Lock Spring system5. High Repeatability
1. Digital reading system
2. High Accuracy (1 – 5 Gal)3. High Repeatability
Graviton
Lacoste & Romberg G with Alliod Sistem
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1. Fully digital reading System
2. Automatic Leveling
3. High Accuracy (1Gal)
4. Automatic Lock Spring system
Scintrex AutoGrav CG-5
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Gravity MeterA gravimeter is any instrument designed to measure spatial variations in gravitational acceleration
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E:\Geophysics Application\Aerogravity.htm
E:\Geophysics Application\gravity.htm
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From EGM96 N-values
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From EGM96 N-values
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Free-air Bouguer
From EGM96 N-values
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Free-air Bouguer
From EGM96 N-values
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Free-air Bouguer
From EGM96 N-values
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Studi Kasus strategi survei 4D gaya berat mikro untuk pemantauan dinamika fluida di permukaan (Sarkowi dkk, 2005).
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SubsidenceSubsidence di dataran di dataran Alluvial Semarang 2003 dari Alluvial Semarang 2003 dari
pengukuran pengukuran levellinglevelling
Anomali Anomali time lapse time lapse microgravitymicrogravity dari Juni 2003 dari Juni 2003
sampai September 2002sampai September 2002
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Model Inversi penambahan air, tidal flood dan subsidenceyang berasal dari Model Inversi penambahan air, tidal flood dan subsidenceyang berasal dari anomaly time lapse microgravity Juni 2003 – September 2002 pada line AB.anomaly time lapse microgravity Juni 2003 – September 2002 pada line AB.
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Anomali yang telah diteliti
proses produksi uap dan injeksi air pada reservoir panas bumi, pemantuan produksi gas, pemantauan produksi minyak yang diinjeksi dengan air atau uap, pemantauan aktifitas gunung api berupa pergerakan magma dan deformasi permukaan.
Penelitian lain
Pemantauan perubahan gaya berat akibat pengaruh hidrologi musiman. Korelasi gaya berat dengan curah hujan. Perubahan kedalaman muka air tanah. pemantauan amblesan di daerah pertambangan juga dapat diteliti dengan metode ini.
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Terlalu banyak koreksi yang digunakan. Ambiguitas.
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Bouguer Gravity of the Lake St. Martin Impact Structure
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Residual Gravity of the Lake St. Martin Impact Structure
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Surprise Valley, California
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Surprise Valley, California
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1400
1450
1500
1550
1600
1650
1700
1750
1800
1850
1900
1950
2000
2050
2100
2150
2200
2250
2300
E levation
Surprise Valley, California
Digital elevation model
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-120.225 -120.215 -120.205 -120.195
41.65
41.655
41.66
41.665
41.67
41.675
41.68
41.685
41.69
41.695
41.7
Surprise Valley, California
Bouguer Gravity contours
Red lines are seismic lines
Black lines are gravity profiles seen in next two slides
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Complete Bouguer Anomaly
Surprise Valley, California
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Surprise Valley, California
Surprise Valley, California
Bouguer gravity profiles
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Surprise Valley, CaliforniaVertical derivative of the Bouguer gravity profiles
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CBA @ 2.4 g/cc with 1 mgal CBA @ 2.4 g/cc with 1 mgal contourscontours
Regional GeologyRegional Geology
B
DP
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Relief on pre-Tertiary basement
Bradys fault zone & possible continuations
High angle faults
CBA @ 2.4 g/cc 1 mgal contoursCBA @ 2.4 g/cc 1 mgal contours
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Desert Peak geothermal aquifer in pre-Tertiary basementDesert Peak geothermal aquifer in pre-Tertiary basementDesert Peak geothermal aquifer in pre-Tertiary basementDesert Peak geothermal aquifer in pre-Tertiary basement
Benoit et al. (1982)
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Gravity pattern correlates with deep drilling intercepts near DP
Shallowest basement on A-A’ is 750 m deep
Basement elevations deepen 100–200 m/km from center of A-A’
Hot Springs Flat basin MZ basement probably deeper than 1400 m
CBA @ 2.4 g/cc 1 mgal contoursCBA @ 2.4 g/cc 1 mgal contours
drilled horstdrilled horst
AA
A’A’
Hot Springs Flat
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Desert Peak cross section
From Faulder and From Faulder and Johnson, 1987Johnson, 1987
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• Reflects a shallow, Reflects a shallow, narrow higher density narrow higher density zonezone
• May be fault boundedMay be fault bounded• May be enhanced by May be enhanced by
hydrothermal mineral hydrothermal mineral precipitation precipitation
• Southern half lacks Southern half lacks known geothermal known geothermal
indicatorsindicators
Vertical Gradient of Gravity FieldVertical Gradient of Gravity Field
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Major high angle faults & contacts
Commonly faults between young basin fill and bedrock
Can localize geothermal fluid flow
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CBA @ 2.4 g/cc 1 mgal contoursCBA @ 2.4 g/cc 1 mgal contours
drilled horstdrilled horst
AA
A’A’
Regional GeologyRegional Geology
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Gravity 1/7
THANK YOU
Question?